Paper coating lubricants are designed to provide improved coating application rheology on blade, air knife and roll type coaters. Lubricants are essential in providing release of the coated surface from the calendars on glossy grades.
New application methods in coating (film coaters and short dwell coaters) and calendering (hot gloss and hot soft nip calendars) have changed the coating application parameters for lubricant utilization. Film coaters and short dwell coaters are generally limited to formulations of 56-58% solids due to low and high shear rheology problems. In film coater applications, an improved release of the wet coating film from the applicator roll is necessary at higher coater speeds to avoid picking and release patterns which are known as "orange peel" and "mottle".
Paper coating lubricants have been grouped into four categories: (1) aqueous dispersions of metallic soaps, (i.e., calcium stearate), (2) wax emulsions (paraffin, microcrystalline and oxidized polyethylene waxes), (3) blends of calcium stearate, a polyethylene glycol fatty ester, urea and possibly polysaccharides, and (4) soy lecithin/oleic acid blends. Each of these products have advantages for certain applications, but all have been found lacking in high-speed or high temperature situations. Additionally, these lubricants display poor rheology under the extremely high shear experienced when paper coating machines run at speeds near or beyond 4500 feet per minute. Also, many paper mills require the chemicals used in their formulations to be on the list of additives approved for use in paper coatings with food contact applications.
Calcium stearate, lecithin/oleic acid and polyethylene lubricants generally increase the viscosity of coatings through chemical interactions with starch, latex, calcium ions and other additives. Importantly, these products have been found to undergo thermal degradation when used on hot gloss or hot soft nip calendars. This degradation manifests itself as "smoking" or "hazing" on the machine in the vicinity of the hot roll.